(1) Field of the Invention
The present invention relates to a halogen lamp with an infrared rays reflective coating, and to a halogen lamp with a reflecting mirror and an infrared rays reflective coating.
(2) Description of the Related Art
With the recent trend toward energy saving, what is called a halogen lamp with an infrared reflective coating (hereinafter, the infrared reflective coating is referred to as an IR coating, and the halogen lamp with an infrared reflective coating is referred to as an IR-coated halogen lamp) has been developed and is becoming widespread. An IR-coated halogen lamp includes an arc tube that has a tungsten filament coil therein, with an infrared reflective coating formed on an outer surface of the arc tube.
As disclosed in Japanese Laid-Open Patent Application No. 10-501368, in the IR-coated halogen lamps, the IR coating formed on an outer surface of the arc tube receive infrared rays from the tungsten filament coil, and reflects them back to the tungsten filament coil. With this construction, the repeatedly reflected infrared rays heat the tungsten filament coil. This reduces the amount of power consumption in the IR-coated halogen lamps, thus improving the lamp efficiency.
The IR coating is basically composed of a plurality of layers including both high-refractive-index interference layers and low-refractive-index layers. With such a construction, on one hand, the IR coating reflects back the infrared rays to the tungsten filament coil, and on the other hand, the IR coating allows the visible rays to pass through itself to outside. Typically, tantalum oxide (Ta2O5) is used as the material of the high-refractive-index interference layers, and silica (SiO2) is used as the material of the low-refractive-index interference layers, for the multi-layered IR coating for halogen lamps.
In recent years, two types of high-efficiency halogen lamps, which are manufactured using the CVD technology, have been on the market: a commercial voltage 100/110V type; and a low voltage 12V type.
In the above-mentioned high-efficiency halogen lamps, the arc tube, which is substantially spheroid and is made of quartz glass, has, on an outer surface thereof, the IR coating which is composed of approximately 20 Ta2O5—SiO2 layers. The tungsten filament coil is deposited inside the arc tube on the central axis thereof. Also, a pair of lead wires are connected to the tungsten filament coil via molybdenum foils. The arc tube is hermetically sealed, and has a single-base structure. The lamp efficiency is as high as 22.4 lm/W and 25.4 lm/W in the cases of the 110V type and 12V type with 50 W, respectively.
It should be noted here that the term “substantially” used in the present document indicates a range that includes a general transposition, deviation or the like.
In addition to the above-mentioned improvement in terms of the lamp efficiency, recently an improvement using an optical system for energy saving has been put into practical use. More specifically, a halogen lamp with a reflecting mirror, which is a combination of a halogen lamp and a reflecting mirror that converges light beams emitted from the tungsten filament coil onto an object, is widely used as a lighting at shops or the like. Also, an IR-coated halogen lamp with a reflecting mirror, which is a combination of an IR-coated halogen lamp and a reflecting mirror, has lately been developed and is becoming widespread.
Among many types of such reflecting-mirror-attached IR-coated halogen lamps, a 12V type emits light beams having a higher luminous intensity than the other commercial voltage types of the same lamp input, in spite of its smallness. Accordingly, the reflecting-mirror-attached IR-coated halogen lamp of 12V type is superior at energy saving to the other types, and is expected to be widespread as a lighting at shops or the like. Among small-scale reflecting-mirror-attached IR-coated halogen lamps of 12V type, main products are a 35 W type and a 50 W type, both with a 50 mm-diameter reflecting mirror.
Meanwhile, the above-mentioned reflecting-mirror-attached IR-coated halogen lamps of a 12V type are generally more expensive than the other types. As a result, the market is demanding a lamp having as long a life as is commensurate with the cost. To meet the demand, the development of a reflecting-mirror-attached IR-coated halogen lamp of a 12V type (with a 50 mm-diameter reflecting-mirror) having a rated life of 4,000 hours has been worked on.
The inventors of the present invention have closely studied the life of the reflecting-mirror-attached IR-coated halogen lamp of 12V type over a considerable period of time. It was found through the study that when the lamps are continuously lighted in a long-term aging test, a serious quality problem occurs after the lamps, especially those of 50W type, are continuously lighted for approximately 3,000 hours. More specifically, it was found that as the lamp is continuously lighted, the temperature of the arc tube rises and a crack is generated in a sealing portion of the arc tube in which the lead wires and the molybdenum foils are embedded. It was also found that in some cases, the arc tube is broken as the sealing portion cracks, and that in rare cases, a front glass attached to the reflecting mirror is also broken by the breakage of the arc tube. Another quality problem was also found. That is to say, after a similar time period has passed during the long-term aging test, the IR coating on an outer surface of the arc tube peels off, which causes the infrared beams from the tungsten filament coil to leak and reduces the amount of light beams emitted from the lamp.
The tungsten filament coil wears as the lamp is lighted, due to vaporization. The life of a halogen lamp ends when the tungsten filament coil is finally broken by the wear. Compared to this, the above-mentioned two quality problems are abnormal since they occur before a lamp life comes to a normal end. Especially, the former problem of the crack in the arc tube should be eliminated with certainty since it concerns the safety during operation of the lamp.
As described above, at present, a main technical challenge regarding a reflecting-mirror-attached IR-coated halogen lamp of 12V type (especially of a high-wattage 50W type with a 50 mm-diameter reflecting mirror) is to find means for preventing the arc tube breakage and IR coating peeling that occur before a lamp life comes to a normal end.